NEW YORK — What kind of deranged people watch “The Silence of the Lambs” and think: That would make a killer musical?

These people: Jon and Al Kaplan, a pair of brothers from New York with a droll sense of humor who work perfectly together, like fava beans and a nice Chianti.

They’ve helped turn one of the creepiest serial killer movies ever made into a 90-minute unauthorized off-Broadway musical, complete with a chorus of dancing lambs running across the stage.

“It all comes from a place of respect,” says Al Kaplan, 32, in the seats of Theatre 80, the space on St. Marks Place where their show is playing. “There’s ‘Naked Gun’-style humor in the show but there’s love for the material everywhere.”

Having won a cult following since some of the songs debuted online nine years ago, “Silence! The Musical” has been expanded into a live stage show with a book by Hunter Bell (who did the same with “title of show”). It made its debut at the 2005 Fringe Festival in New York and last year was staged in London.

The show, which follows the movie quite faithfully, stars Brent Barrett as Dr. Hannibal Lecter, a brilliant psychiatrist-turned-serial-killer who matches wits with Clarice Starling, an FBI trainee played by Jenn Harris.

Leaving obscure clues, Lecter helps Starling search for “Buffalo Bill” — an unknown serial killer who we learn likes to starve his victims so he can harvest their skin and sew it into attractive clothing items.

“The general story is the same. We push elements that are there further,” says Jon Kaplan, 35. Adds his brother: “It’s all about exaggerating certain things — like accents and lisps.”

Some of their more than a dozen songs include “Are You About a Size 14?” sung by Buffalo Bill which includes the lyrics: “I’ve got her in my sights/She’s appropriately fat/I’ll wait for her to notice me/I hope she fed her cat.”

There’s also “It’s Me,” a duet between Lecter and the police that culminates in a classic — and deeply gross — film moment. “This cop is already dead/You’ll see/I’m wearing his face on my head/It’s me.”

The Kaplans, raised on Staten Island but who moved to Los Angeles in 1996 to study concert composition at the University of Southern California, loved the original film, based on the best-selling book by Thomas Harris, and committed many of the lines to memory.

They were especially taken by the character of Buffalo Bill, who in his strange voice orders his victim to rub lotion on their skin. “Years and years later we would randomly be saying Buffalo Bill lines for no reason. It had just entered our vernacular,” says Al Kaplan.

The brothers, who split lyric and song writing duties, first began work on the “Silence” project in 2002 when they created nine parody songs and put them on the Web as a joke. The audio tracks soon went viral — hard to do in the days before YouTube and Facebook.

Movie producers eventually approached and suggested they turn their songs into a movie, which the Kaplans thought was a very lousy idea. They wrote a screenplay anyway and when theater producers came knocking they had a rough story waiting.

Over the years, the brothers came very close to having their show make its off-Broadway debut but squeamish producers dragged their heels. This year, which marks the 20th anniversary of “The Silence of the Lambs,” they’ve had better luck, in part they think because shows such as “The Book of Mormon” have pushed the envelope of what a musical can discuss.

“In recent years, it seems vulgarity is back a little bit in theater. When we did this in 2005, it brought out some outrage in people,” says Al Kaplan. “Now maybe this will be tame in comparison.”

The brothers, who work as film and TV composers, have also done parodies on the side of Arnold Schwarzenegger movies such as “Conan the Barbarian,” “Commando” and “Predator,” as well as “Schindler’s List: The Musical.”

“We prefer to take something that’s not a comedy and turn it into a musical. That sort of inherently adds a little layer of humor to it even if you’re not trying to write a funny song,” says Jon Kaplan.

Though they are not aware that anyone from the movie has ever seen their version of “The Silence of the Lambs,” Jon Kaplan says he would love it if one day Anthony Hopkins, who played Lecter, stopped by.